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This Week in Science

The Big Squeeze

During the past decade, the volcanic activity of Mount Etna has been recorded with a dense network of instruments. Patanè et al. (p. 2061) have combined recent tomographic models with refined seismic and geodetic data to define the dynamics and extent of magma below the 3- to 5 kilometer-deep shallow reservoir. Their work suggests that from 1994 to 2001, magma was filling a complex conduit of dikes and sills at depths of 6 to 15 kilometers. This complex conduit was then radially compressed, which forced magma into the shallow reservoir and caused the intense activity that started in October 2002.

CREDIT: PATANÈ ET AL.

Thriving Under Soft Light

Plants use diffuse radiation more efficiently than direct sunlight, so did the increase in diffuse radiation caused by volcanic aerosols injected into the atmosphere from the 1991 eruption of Mount Pinatubo enhance terrestrial photosynthesis? Gu et al. (p. 2035) examined the photosynthetic activity of a northern hardwood forest in 1992 and 1993 when a sharp decline in the growth rate of atmospheric CO2 was observed. They provide evidence linking the Pinatubo eruption to an enhanced terrestrial carbon sink caused by an increase in photosynthesis, not by a decrease in ecosystem respiration.

Casting Call for Sensors

Dedicated optical sensors often rely on filters to select particular wavelengths of interest. One type of filter, the rugate structure, takes advantage of continuous changes in refractive index of a material to create desirable optical interferences. Rugate filters can be fabricated from porous silicon, a material in which changes in porosity and hence refractive index are readily achieved. Li et al. (p. 2045) show that polymer replicas of porous silicon structures also act as rugate filters. These filters can be used for vapor sensing and as self- reporting biodegrading structures for drug delivery.

Oscillating Magma

Magmatic movement in conduits or cracks beneath volcanoes displays a characteristic seismic signature—harmonic oscillations with periods of 0.2 to 2 seconds (s) that decay within 60 s. Kumagai et al. (p. 2058) have used broadband seismic instruments on Mount Hachijo Fuji, Japan, to record harmonic oscillations with periods of about 10 s that decay within 300 s. These very-long-period events followed a seismic swarm and surface deformation of the summit of the volcano. A model of these events suggests that a 4-kilometer-long vertical crack about 5 kilometers beneath the summit fills with basaltic magma that resonates when excited by volcano-tectonic activity.

Positive Interference

RNA interference (RNAi) is now often used to block gene expression and create a phenotype that hopefully gives clues into the gene's function. Lum et al. (p. 2039; see the news story by Couzin) have developed a high-throughput genome-scale screen that uses RNAi and cultured Drosophila cells to identify new components of a specific signaling pathway rapidly and systematically. The method identified several new positive and negative regulators of the Hedgehog signaling pathway, some of which were only associated with the Wingless signaling pathway. This RNAi-based approach to functional genomics may allow a more complete view of signaling mechanisms and regulatory cross-talk between pathways.

CREDIT: LUM ET AL.

Aluminum Hydride in Isolation

Boron hydrides comprise a large family of rings and cage compounds that form through hydrogen bridge bonds. The bonding of aluminum hydrides is more poorly understood because it tends to form either extended polymers in bulk or only isolated AlH3 groups in matrices. Andrews and Wang (p. 2049) report that Al2H6 can be isolated if hydrogen itself is used as the matrix. Vibrational spectra show that Al2H6 is indeed isostructural with B2H6 and adopts a dibridged geometry.

Lessons from Leviated Liquids

Hydrodynamic theory treats liquids as a continuum, whereas kinetic theory treats liquids on shorter atomistic length scales. In an effort to provide experimental constraints at the boundary between these two theories, Sinn et al. (p. 2047; see the Perspective by Kieffer) studied collective excitations, such as sound waves, in liquid aluminum oxide, a molten salt. They could levitate the samples without containers and perform high-resolution inelastic x-ray scattering measurements. The liquid could be described by hydrodynamics with frequency-dependent viscosity or by kinetic theory with more rigid-body behavior.

Compounding Uncertainties

Predictions of future global warming from fossil fuel burning span a wide range of positive values because there is a considerable uncertainty about how sensitive climate is to the effects of various kinds of air pollution. An analysis by Caldeira et al. (p. 2052) on power generation strategies shows that a factor of 3 uncertainty in the effect of CO2 on climate sensitivity translates into a factor of 8 uncertainty in allowable increases in atmospheric CO2 concentration, and even greater uncertainty in allowable CO2 emissions. Unless climate sensitivity is low and acceptable amounts of climate change are high, climate stabilization will require a massive transition to carbon-emission-free energy technologies.

From Herbicides to Human Therapeutics

Acetyl-coenzyme A carboxylases (ACCs) are critical enzymes in the fatty acid biosynthetic pathway and are targets for drugs against obesity and diabetes. Several herbicides inhibit the carboxyltransferase (CT) domain of plant ACCs. Now Zhang et al. (p. 2064) have determined the structure of the CT domain of yeast ACC at 2.7 angstrom resolution. The domain exists as a dimer, and each monomer comprises two subdomains that have a backbone fold similar to other enzymes involved in fatty acid degradation. The active site is at the dimer interface. The structure, together with mutagenesis and kinetic studies, shows that herbicides target the active site of CT, thus providing a lead for development of therapeutics.

A Sense of Direction

Odorant receptors can be found outside of the olfactory system, including the mammalian testis. Spehr et al. (p. 2054; see the Perspective by Babcock) identified and characterized a previously unknown odorant receptor, hOR17, in human sperm. They identify ligands and antagonists for hOR17 that affect the swimming speed and direction of human sperm. Because chemotactic responsiveness and fertility may be related, this work may contribute to research to understand infertility and develop contraceptives.

Genomes We Carry with Us

The genome sequences of two gastrointestinal bacteria, one a human symbiont and another an infectious agent, reveal insights into how they can interact with or against us (see the Perspective by Gilmore and Ferretti). The genome of Bacteroides thetaiotaomicron, an important human gut symbiont, has been sequenced by Xu et al. (p. 2074). Numerous gene products are devoted to acquiring and breaking down dietary plant polysaccharides that humans cannot otherwise digest. Enterococcus faecalis V583 causes antibiotic-resistant infections, including resistance to the last bastion of infection control—vancomycin. Paulsen et al. (p. 2071) sequenced its genome and found adaptations for coping with tough environments. They also found an extraordinarily high proportion of mobile elements, some of which may be involved in the onward transfer of vancomycin resistance to other pathogens.

Catching the Intermediates in Phosphoryl Transfer

Numerous enzyme-based processes capture the energy contained within the β-γ-phosphoanhydride bond of ATP by using a water molecule to liberate inorganic phosphate from adenosine diphosphate. This reaction proceeds via nucleophilic attack by the water and a transition state centered on a pentavalent phosphorus. Many regulatory processes rely on protein kinases, which transfer the terminal phosphoryl group from ATP to amino acid acceptors (serine, threonine, and tyrosine) through a similar trigonal bipyramidal transition state. Lahiri et al. (p. 2067; see the Perspective by Knowles) present an atomic-resolution snapshot of an enzyme-stabilized pentavalent phosphorus, a high-energy intermediate in the phosphoryl transfer reaction catalyzed by phosphoglucomutase.

CREDIT: KNOWLES

Revealing Flaws in Immune Defenses

In response to innate inflammatory signals, Toll-like receptors and interleukin-1 (IL-1) receptors activate the NF-Bκ and p38-MAP kinase pathways by recruiting the IL-1 receptor-associated kinase (IRAK) to their intracellular Toll-IL-1 receptor domains. IRAK-4 deficiency in mice leads to a profound immune deficiency to a range of bacterial and viral infections. Picard et al. (p. 2076) describe three patients with inherited mutations in the IRAK-4 gene that lead to impaired immunity of certain common species of pyogenic bacteria. However, unlike the situation in mice, human IRAK-4 appears to be redundant in coping with other types of viral, bacterial, and fungal infection.